This invention relates to fiber optic devices such as fiber gratings and more particularly to a package and system to compensate for the thermal dependence of such devices.
1. Background
The Bragg effect is employed in optical communications systems for, amongst other things, wavelength selective filtering. In this implementation the filter is used in add/drop wavelength applications and in multiplexing and demultiplexing functions. Bragg filters are also used in Mach-Zehnder interferometer applications for various optical communication related functions.
A grating is a series of perturbations in an optical wave guide precisely positioned according to a desired wavelength effect. It is known that such gratings are thermally dependent wherein the spacing between perturbations and the refractive index of the waveguide materials actually increase with increasing temperature. This temperature dependence, if not compensated for, will change the effective central wavelength of the grating as a function of operating temperature.
2. Prior Art
There are known methods of incorporating thermal compensation strategies into fiber optical devices. The first such method to be described here involves a package consisting of a holding tube and a pair of threaded, smaller tubes designed to fit within the holding tube. The holding tube is made of a material that has a different coefficient of thermal expansion (CTE) than that of the threaded tubes. The grating is fixed to the smaller tubes in such a way that it is strained by an amount designed to compensate for its temperature dependence when the temperature changes. Strain arises because of the different coefficients of thermal expansion of the two kinds of tubes. U.S. Pat. No. 5,914,972 which issued Jun. 22, 1999 to Siala et al. describes one such package. U.S. Pat. No. 5,042,898 which issued Aug. 27, 1991 to Morey et al. describes a similar arrangement and includes discussion regarding the thermal dependence of a grating.
A second solution consists of fixing the grating, whether it be a fiber Bragg grating or a Mach-Zehnder interferometer, to a substrate and then gluing the substrate to a bi-metal plate. The bi-metal plate is composed of two materials, each with a different coefficient of thermal expansion, sandwiched in such a way that when the temperature changes the bi-metal plate bends. The bending of the bi-metal plate induces a strain on the substrate affixed to it which is proportional to the length of the bi-metal plate. It is this strain which compensates for the temperature dependence of the grating. U.S. Pat. No. 5,978,539 which issued Nov. 2, 1999 to Davis et al. describes a variant of this concept.
A third approach consists of fixing the fiber Bragg grating or Mach-Zehnder interferometer to a special substrate that has a negative coefficient of thermal expansion of exactly the correct value so that it shrinks by just the right amount to compensate for the thermal variation of the spectral response of the device. U.S. Pat. No. 5,926,599 which issued Jul. 20, 1999 to Bookbinder et al. gives one example of this approach.
There is a further approach described in International application WO 00/54082 published Sep. 14, 2000 to Maaskant et al. that describes a shaped substrate that is designed to bend in a controlled fashion in response to temperature variations. The fiber device is attached to the substrate in such a way that the bending action changes the amount of tension on the fiber device in response to temperature changes.
Another compensation technique is described in Applicant""s co-pending U.S. application Ser. No. 09/985,041 filed Nov. 1, 2001 the contents of which are incorporated herein by reference. The compensation technique of the earlier application is based on a modification of the aforementioned bi-metal approach whereby the use of glue to hold the substrate to the bi-metal strip is rendered unnecessary. According to this prior design a bi-metal element comprising two components is used. Instead of gluing a bi-metal plate to the fiber device substrate, the substrate itself is used as the first component of the bi-metal element and is shaped in such a way that the second component of the bi-metal element forces it to curve by pushing against it when subject to a temperature increase. The curvature of the first component of the bi-metal element changes the strain state of the fiber attached to it. The main component of the force acting to curve the fiber device is therefore held mechanically instead of relying on the sheer strength of a glue.
In the aforementioned solution the combination of materials with different coefficients of thermal expansion effectively attain a net negative CTE of a generally proper size. The net CTE obtained is a function of the CTE of the individual materials and of their sizes. Therefore it is adjustable. As indicated previously an alternative approach consists of fixing the fiber device to a material with a negative CTE.
The solution based on materials with different CTE works well but the package needs to be either significantly longer than the fiber device it is designed to compensate or involves the bending of interconnected parts, which is often difficult to implement. Access to the fixing points (glues or solder) is also difficult. Solutions based solely on negative CTE materials solve these problems but introduce an additional one in the sense that the effective CTE is an inherent property of the materials and thus it is impossible to incorporate a fine adjustment for different optical fiber devices. Furthermore, the negative CTE of such materials tends to vary from batch to batch.
The present invention consists of using a material with a CTE that is excessively negative and to combine it with a positive material with adjustable length in such a way as to provide an adjustable, negative CTE. In this way, the length of the package need not be much longer than the device to be compensated and most of the advantages associated with using negative CTE materials are preserved while avoiding their main disadvantage (lack of adjustability).
Therefore, in accordance with a first aspect of the present invention there is provided a temperature compensating package for an optical fiber device comprising: a support structure of material having a negative CTE; and securing means located in the support structure for securing opposed ends of an optical fiber device, at least one of the securing means being of a material having a positive CTE and adjustable lengthwise.
In accordance with a second aspect of the present invention there is provided a method of providing thermal compensation to an optical fiber device comprising: providing a support structure for the optical fiber device, the support structure being of a material having a negative coefficient of thermal expansion and having securing means at each end, at least one of the securing means being of a material having a positive coefficient of thermal expansion and adjustable lengthwise; adjusting the at least one securing means to establish a base thermal compensation value; and securing the optical fiber device to the securing means within the support structure.
In accordance with a third aspect of the present invention there is provided a fiber optical device assembly with associated thermal compensation comprising: an optical fiber having a fiber optical device therein; a support structure of a material having a negative coefficient of thermal expansion through which the optical fiber extends; securing means in each end of the support structure and securing the optical fiber on opposite ends of the optical fiber device, at least one of the securing means being of a material having a positive coefficient of thermal expansion and being adjustable longitudinallly of the optical fiber.